在抽象类中自动装配 null

Question

I had an issue to autowire an object in my abstract base class. It always give me null instead of an instance. Please help.

Base class:

public abstract class BaseClass implements IReq<Req> {

    @Autowired
    protected ReqDao dao;

    protected void updateReq() {
         dao.update();
    } 
}

Child class:

@Component
public class ChildClass extends BaseClass {
    ...
}

ReqDao class:

@Component
public class RptRequestDao {
     public void update(){
          ...
     }
}

I am thinking of simply use the update() function in Base class, which means in my ChildClass, I don't override that one. Is this the problem? If it is, what's the normal way to do it? Thanks in advance.

Answer 1

A bean is created on demand, in your case when you initialize your object, RepoDao is private so it wont be inhereted to the class which will be intialized, you either need to put

@Component
public class ChildClass extends BaseClass {
    @Autowired
    private ReqDao dao;

or make it protected/public in BaseClass, sure public will make it accessible to other classes which violates the encapsulation

Answer 2

Instantiate your child class using @Autowired like this

public class SomeClass{

@Autowired
ChildClass childClass; //IMPORTANT

}

The error occurs when you try to instantiate the childClass like this:

public class SomeClass{

ChildClass childClass = new ChildClass();

}

Answer 3

BaseClass 是抽象的，你不能实例化它。你需要在 ChildClass 中让你的 ReqDao 被自动装配。Spring 只会在它创建该类的实例时自动装配。希望有帮助

Answer 4

Check how you are instantiating the child class. Make sure you are NOT using "new" keyword to instantiate child class.

ChildClass cc = new ChildClass() // **Not recommended**

You must autowire the childclass and let spring take care of bean creation.